(1) Field of the Invention
This invention relates generally to voltage converter circuits using charge pumps and relates more particularly to the usage of an active circuitry to discharge all capacitors employed by such a charge pump.
(2) Description of the Prior Art
Charge pumps are circuits that generate a voltage larger than the supply voltage from which they operate. Charge pumps are a form of DC-DC converters that rely on capacitors instead of inductors for energy storage and transfer. The absence of inductors makes them attractive in situations requiring a low-power auxiliary supply. They use less circuit-board area, offer minimal component height, and are easy to use.
Charge pumps are switching capacitors circuits. The basic circuit is made from switches and capacitors. Charge pumps work as fixed multipliers, positive or negative, of the supply voltage. In order to generate twice the supply voltage two capacitors are required. One capacitor is a “flying” capacitor actually doing the pumping and one capacitor is acting as a reservoir, holding the generated voltage.
In order to generate three times the supply voltage two flying capacitors are required and one reservoir capacitor, in order to generate four times the supply voltage three flying capacitors are required and one reservoir capacitor, etc. for five times the supply voltage.
Charge pumps can have regulated or unregulated outputs. An unregulated charge pump either doubles or inverts the voltage that powers it and the output voltage is a function of the supply voltage. A regulated charge pump either boosts or inverts the supply voltage. Its output voltage is independent of the supply voltage.
Active discharge circuits in voltage charge pumps are normally used to shut down the circuit or reduce the output voltage to an initial value.
Especially for charge pumps having multiple stages it is important to control the discharge process to avoid excessive voltage on the capacitors during a shutdown of the charge pump.
There are patents known dealing with the control of the discharge of capacitors:
U.S. Pat. No. (6,392,904 to Bayer et al.) describes a DC/DC converter including a charge pump circuit comprising one or more capacitors and a plurality of controllable switches connected thereto, the controllable switches being controllable by a control circuit so that the capacitors is/are alternatingly switched in a charging and discharge phase; a first current source set to a predetermined base current located either in the discharge or charging path of the charge pump circuit and a second current source connected in parallel thereto; and a regulator circuit for generating a first control signal representing the difference between a voltage characterizing the output voltage and a first reference voltage and controlling the second current source when the charge pump circuit is active so that the controllable current is reduced or increased with an increase and reduction respectively in the difference to track the voltage characterizing the output voltage in accordance with the first reference voltage; and for generating a second control signal guided to the control circuit, this signal assuming a first status when the voltage characterizing the output voltage exceeds a second reference voltage at a predetermined level above the first reference voltage, upon which the control circuit deactivates the charge pump circuit, and assumes a second status when the voltage characterizing the output voltage drops below the second reference voltage, upon which the control circuit activates the charge pump circuit.
U.S. Pat. No. (6,483,282 to Bayer) discloses a charge pump-type DC/DC converter comprising n elementary stages, each consisting of a charge pump capacitor and several controllable switches connected thereto, whereby the input voltage of the DC/DC converter is applied to the input of the first stage, both electrodes of the charge pump capacitor of the k.sup.th stage are each connectable to one of the controllable switches with the output of the (k−1).sup.th stage, k=2, . . . , n and the output of the n.sup.th stage forms the output of the DC/DC converter. The DC/DC converter in accordance with the invention is characterized in that it in addition enables one or more further controllable switches to be connected, via which the electrode of the charge pump capacitor of the n.sup.th stage which in the discharge phase is not connected to the output of the converter, to one or more outputs of the 1.sup.th stage (1=(n−2), . . . 1) and/or of the input voltage, and comprises a control circuit which in the discharge phase of the charge pump cycle signals ON, as a function of the input voltage of the DC/DC converter, that switch of the array consisting of the one controllable switch via which the electrode of the charge pump capacitor of the n.sup.th stage can be connected to the output of the (n−1).sup.th stage and the further controllable switches and connects the voltage applied thereto to the cited electrode of the charge pump capacitor of the n.sup.th stage at which the efficiency of the DC/DC converter is a maximum.
U.S. Pat. No. (6,597,156 to Gogolla) discloses an arrangement and a method for generating high voltage from a low-voltage supply source using a charge reservoir chargeable and rechargeable to the desired high voltage or higher via a charge pump. A first switching device regulates the supply of electric charge from the precharged charge reservoir to a separate storage capacitor. As a result, a high voltage signal that is extensively noise-free is available during certain operating phases, such as a measurement process. A second switching device ensures a regulated discharge from the storage capacitor when the capacitor is overcharged, for example, for the readjustment of the high voltage to be generated. A capacitive voltage divider, which reduces the high voltage without losses, is provided to determine the actual value of the high voltage. At very small currents, the desired high voltage can be maintained by using the capacitive voltage divider for a substantially longer time than by using an ohmic voltage divider.